The invention relates to a process for evaporating liquid oxygen and to its use in a process for producing oxygen by low-temperature fractionation of air.
Oxygen, in the present application, is taken to mean any mixture which has an oxygen content elevated with respect to air, for example at least 70%, preferably at least 98%. (In this application, all percentages denote molar amounts, unless explicitly stated otherwise.) This includes, in particular, impure oxygen, and also industrial-grade pure oxygen and high-purity oxygen having a purity of 99.99% or above. For a host of applications, it is necessary to convert liquid oxygen present, before its use, into the gas form by evaporating it in a main evaporator by indirect heat exchange with a heat carrier.
An evaporation of this type occurs in particular in the production of gaseous oxygen by low-temperature rectification, in which the oxygen product occurs in the liquid state at the bottom of a rectification column, since it is less volatile than nitrogen and argon. To obtain the product in the gas form and to generate ascending vapour for the rectification column, the oxygen occurring in the liquid state must likewise be evaporated in a main evaporator. The most widespread here is the classic Linde double-column process in which the main evaporator is disposed in the bottom of a low-pressure column and is operated by condensing nitrogen from the top of the pressure column (see Hausen/Linde, Tieftemperaturtechnik [low-temperature engineering], 2nd Edition, Section 4.1.2 on page 284). The main evaporator in this case is operated as a condenser-evaporator and is frequently termed main condenser. It is also implemented by one or more heat-exchange blocks which are operated as circulating or falling-film evaporators.
The invention also relates to other double-column processes in which the main evaporator is operated with air, for example, and also processes having three or more columns for nitrogen-oxygen separation. Downstream of the rectification column or columns for the nitrogen-oxygen separation, apparatuses for producing other air components, in particular noble gases, can be connected, for example for argon production.
If liquid oxygen is evaporated completely or essentially completely, less volatile impurities, such as CO2 or N2O, for example, can accummulate in the evaporator, even if these impurities are only present in very low concentrations in the oxygen (or the air to be fractionated) to be evaporated. (However, the acetylene which was feared earlier is no longer a problem in air-fractionation plants having preliminary purification by adsorption). Some of these less volatile substances, CO2 and N2O, for example, can precipitate as solids and must be removed from time to time so that blockage of the heat-exchange passages in the main evaporator is avoided. To remove these solids which have separated out, the entire plant must be shut down. In a large air-fractionation plant, this can mean a works shutdown of from two to five days, for example.
To reduce the accummulation of less volatile components, it is customary to take off continuously, or from time to time, some liquid in the form of a purging stream from the main evaporator and to discard this stream. Together with this purging volume, the less volatile impurities accummulated in the oxygen which has remained in the liquid state are also removed, so that their concentration can be limited in the main evaporator. In an air-fractionation plant having preliminary purification by adsorption, the purging volume is customarily from 0.02 to 0.04% of the total amount of liquid oxygen introduced into the evaporator. Since, for air purification upstream of the rectification, molecular sieve absorbers have been used instead of the reversible heat exchangers (Revex) or regenerators previously used, the problems due to the accummulation of combustible less volatile components in an oxygen evaporator of this type (main evaporator) have decreased to the extent that a purging stream of this type is sufficient to prevent hazardous concentrations of hydrocarbons without requiring additional measures (see Hausen/Linde, Tieftemperaturtechnik [low-temperature engineering], 2nd Edition, Section 4.5.1.5 on pages 312 and 313).
The object underlying the invention is to increase the availability of a main evaporator for evaporating liquid oxygen and, in particular, to prevent interruptions to operations as far as possible.
This object is achieved by the features of Patent Claim 1. In this, the (first) purging stream which is taken off from the main evaporator is passed into an auxiliary evaporator which is disposed separately from the main evaporator. In this auxiliary evaporator a large part of the first purging stream is evaporated and can thus be produced as oxygen product or as intermediate oxygen product. In turn, a second purging stream is taken off from the auxiliary evaporator and discarded. (In the special case that krypton and/or xenon are to be produced from the liquid oxygen, further work-up is necessary.) Whereas the first purging stream is continuously passed from the main evaporator to the auxiliary evaporator, the second purging stream can be taken off continuously or batchwise.
In the invention, a relatively large amount of liquid can be taken off from the main evaporator as first purging stream, so that all of the less volatile components can be ejected and their concentration can be kept low in the main evaporator. In particular, no solids deposits occur either in the main evaporator. However, this large volume of purging liquid is not completely lost, since some of the first purging stream is evaporated in the auxiliary evaporator and taken off in the gas form. From the auxiliary evaporator, merely a customary purging volume is taken off as second purging stream, for example from 0.02 to 0.5%, preferably from 0.02 to 0.2%, of the amount of liquid oxygen introduced into the main evaporator. (In the case of batchwise taking off of the second purging stream, the percentages refer to the time average.) The remainder of the first purging stream is evaporated in the auxiliary evaporator and can be utilized as gaseous oxygen product.
Using the invention it is possible to purge the main evaporator so intensively that the content of less volatile components which could lead to solids deposits is kept extremely low. The less volatile components are passed completely to the auxiliary evaporator and there removed via the second purging stream and the heating operation performed from time to time.
Solids deposits can therefore occur only in the auxiliary evaporator, but not in the main evaporator. However, the auxiliary evaporator can be freed from solids considerably more simply than the main evaporator by heating. For this purpose, the normal operation is occasionally interrupted by a heating operation, in the heating operation the auxiliary evaporator being separated from the main evaporator with no liquid being passed from the main evaporator into the auxiliary evaporator. Simultaneously, the auxiliary evaporator is brought to a temperature which is markedly higher than its temperature in the normal operation, for example by at least 20 K, preferably from 20 to 50 K. The operation of the main evaporator and the plant in which it is installed does not need to be interrupted in this process. Due to the intensified purging of the main evaporator, this no longer needs to be heated to remove solids.
It is expedient if the amount of the first purging stream which is taken off from the main evaporator in the normal operation is at least 1%, preferably at least 3%, and/or at most 10%, preferably at most 5%, of the amount of liquid oxygen introduced into the main evaporator.
The invention further relates to the use of the process according to Claim 1 or 2 in a process for the low-temperature fractionation of air according to patent Claim 3 and, in a corresponding apparatus according to Patent Claim 6, in particular air-fractionation processes and plants having air prepurification by adsorption, for example on a molecular sieve. Processes and plants of this type serve for the production of oxygen, nitrogen and/or other gases present in atmospheric air.
In addition, the invention relates to an apparatus for evaporating liquid oxygen according to Patent Claims 4 and 5.